Method for energy dither of a particle beam

ABSTRACT

A method for varying the wavelength of a free electron laser (FEL) by applying an energy dither to the charged particles supplying the FEL. Bunches of charged particle beams are accelerated by cavities that are operated at a harmonic of the bunch repetition rate. The method involves adding one or more secondary radiofrequency accelerator cavities after the primary beam transport and near the wiggler to apply a fluctuation between individual bunches with a pseudo-random distribution. The secondary radiofrequency accelerator cavities provide fine variations of the beam energy about a nominal operating point. Operating a free electron laser (FEL) with a 1% change in the electron beam energy via the added secondary cavities will result in a 2% wavelength variation of the FEL output.

This application is a divisional application of U.S. patent applicationSer. No. 14/719,380 filed on May 22, 2015 and claims the benefit ofProvisional U.S. Patent Application Ser. No. 62/001,656 filed May 22,2014.

The United States Government may have certain rights to this inventionunder Management and Operating Contract No. DE-AC05-06OR23177 from theDepartment of Energy.

FIELD OF THE INVENTION

The present invention relates to charged particle beams and moreparticularly to a method for applying an energy dither to a chargedparticle beam in order to vary the wavelength of the laser lightproduced by the charged particle beam.

BACKGROUND OF THE INVENTION

The output wavelength of a Free Electron Laser (FEL) is determined bythe electron beam energy and the wiggler or undulator parameters ofwavelength and field strength. It may be desirable in certainapplications to vary the wavelength of the FEL output. Although thiscould in principle be done by changing the wiggler parameters, suchvariation would occur too slowly for many applications.

Although the electron beam energy out of the accelerator can be changedby the primary acceleration system, the physics of the electron beamtransport from the accelerator to the wiggler may be affected in anundesirable manner which degrades or prevents the lasing process.

Accordingly, it would be desirable to provide a method for varying thewavelength of the output of an FEL in a manner that does not degrade orprevent the lasing process.

OBJECT OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor applying an energy dither to a charged particle beam in order tovary the wavelength of the laser light produced by the charged particlebeam.

A further object of the invention is to provide a fluctuation betweenindividual bunches of charged particles with a pseudo-randomdistribution.

A further object is to provide a distribution of laser wavelengths in afree electron laser.

A further object is to provide a means of varying the energy in a freeelectron laser in order to vary the output wavelength of the FEL by adetermined amount.

Another object of the invention is to provide a method for achievingfine variations of the beam energy of a charge particle beam by addingone or more separate radiofrequency accelerator cavities near thewiggler after the primary beam transport.

A further object of the invention is to provide a method for varying theenergy output of a free electron laser to produce a small number ofunique values that are reproducible in time by adding one or moreaccelerating cavities in which the frequency of the cavities are set ata harmonic or sub-harmonic of the primary accelerator bunch frequency.

A further object of the invention is to provide one or more acceleratingcavities to a free electron laser near the wiggler after the primarybeam transport and to operate the cavities at a non-integer harmonic orsub-harmonic of the primary accelerator bunch frequency in order to varythe output in a pseudo-random manner over a range set by the cavityaccelerating gradients and frequencies.

SUMMARY OF THE INVENTION

The invention is a method for applying an energy dither to a chargedparticle beam in order to vary the wavelength of the charged particlebeam. Bunches of charged particles are accelerated by cavities that areoperated at a harmonic of the bunch repetition rate. One or moresecondary radiofrequency accelerator cavities are added near the wigglerafter the primary beam transport to apply a fluctuation betweenindividual bunches with a pseudo-random distribution. The secondaryradiofrequency accelerator cavities provide fine variations of the beamenergy about a nominal operating point. Operating a free electron laser(FEL) with a 1% change in the electron beam energy via the secondarycavity will result in a 2% wavelength variation of the FEL output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of the energy distribution of a particle beam subjectto an applied energy dither according to the present invention.

FIG. 2 is a schematic depicting the layout of a free electron laserillustrating the principles of the present invention.

DETAILED DESCRIPTION

The current invention includes a method for applying an energy dither toa charged particle beam in order to provide fine variations of the beamenergy about a nominal operating point.

According to the method of the present invention, one or more secondaryradiofrequency accelerator cavities are added near the wiggler after theprimary beam transport. The secondary radiofrequency acceleratorcavities are used for fine variations of the beam energy about a nominaloperating point.

In a practical application, with reference to FIG. 2, depicting a freeelectron laser 12 supplied with charged particles from a primaryaccelerator 20, one or more secondary radiofrequency acceleratorcavities 10 a and 10 b are added to a free electron laser (FEL) 12 nearthe wiggler 14 after the primary beam transport 16. The wiggler 14includes a group of magnets 15 with alternating poles longitudinallyarranged along a narrow gap. The frequency of the secondary acceleratingcavity may be chosen as a harmonic or sub-harmonic of the primaryaccelerator bunch frequency, in which case the variation in energyoutput will be a small number of values reproducible in time. If a morerandom variation is desired, the cavity frequency or frequencies can bechosen as non-integer harmonic, harmonics or sub-harmonics of the bunchfrequency and the output will vary in a pseudo-random manner over arange set by the cavity accelerating gradients and frequencies. A widerange of energy distributions can be obtained depending on theseparameters. An approximate 1% change in the electron beam energy via thesecondary cavity 10 will result in a 2% wavelength variation of the FELoutput 18.

As an example, if one were to use 320 MHz for the bunch frequency, asecondary cavity 10 a operating at 833.23 MHz with an acceleratingvoltage of 0.88 MeV, a secondary cavity 10 b operating at 823.0 MHz withan accelerating voltage of 1.0 MeV, and sample the resultant beamvariation over a 1 ms period; one would obtain 7745 energies with thedistribution functions shown in FIG. 1.

The method of the present invention, using cavities with frequenciesthat are non-integer harmonics of the bunch repetition rates, allows oneto vary the wavelength of the FEL output beam by changing the energy ofthe input particle beam, which is valuable in industrial applications ofFELs and other charged particle beams. The method described herein isapplicable to any charged particle beam, including electron beams andproton beams, and is also applicable to any architecture for producingthe beam including free electron lasers, synchrotrons, cyclotrons, andlinear accelerators. The meaning of various terms used herein are asfollows:

1) The term “primary accelerator bunch frequency” as used herein refersto the primary frequency applied to group electrons such that theyoccupy a small longitudinal space relative to the wavelength of theaccelerating RF frequency. The CEBAF bunch length is about 0.5 mm. Thenumber of electrons per bunch can be calculated from the beam current,the electron charge (1.60217733×10⁻¹⁹ C), and the bunch frequency(nominally 1497 MHz).

2) The term “wiggler” refers to a group of magnets with alternatingpoles longitudinally arranged along a narrow gap in order to bend aparticle beam in a sinusoidal path to generate synchrotron light.

3) The term “radiofrequency accelerator cavity” refers to a closedvolume structure, e.g. cube, cylinder, sphere, ellipsoid, that resonatesat frequencies whose wavelengths are half integer multiples of thedimensions. At the right frequency, a resonant field can build up tostore thousands (copper cavity) or millions (superconducting cavity) oftimes more energy than when off resonance. This is the fundamentalfoundation for accelerating structures that can develop gradientsequivalent to millions of volts.

4) The term “primary beam transport” as used herein refers to the systemused to control the particle beam path and the energy properties of thebeam in order to transport the beam to the wiggler.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiments herein were chosen and described in order to best explainthe principles of the invention and the practical application, and toenable others of ordinary skill in the art to understand the inventionfor various embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method for varying the output wavelength of afree electron laser by changing the energy of the input particle beam,comprising: a. providing an accelerator producing a charged particlebeam including bunches of charged particles having a primary bunchrepetition frequency at a primary accelerator cavity frequency or asubharmonic thereof; b. providing a wiggler to periodically laterallydeflect the charged particle beam; c. adding one or more secondaryradiofrequency accelerator cavities after the accelerator and before thewiggler, d. setting the frequency of each added secondary radiofrequencyaccelerating cavities to a non-integer harmonic or subharmonic of theprimary bunch repetition frequency; and e. adjusting the acceleratingvoltage of each of the secondary cavities to provide a desired variationin the energy of the bunches which make up the charged particle beam,whereby the variation in the output wavelength of the free electronlaser includes a distribution equal to a multiple of the energydistribution of the charged input particle beam.